By and large, a power supply utilizes an AC/DC converter to receive the commercial AC power and transforms the AC power to a DC power with a high-voltage level. Subsequently, a DC/DC converter is used to convert the DC power with a high-voltage level into a DC power with a low-voltage level for operating an electronic device, for example a desktop computer or a notebook computer.
Part of the circuit layout of the conventional power supply is as shown in FIG. 1. In FIG. 1 the power supply 10 is composed of a main circuit 11, a discharge circuit 12 and a high-voltage detecting circuit 13. The main circuit 11 contains a PFC IC 111. The high-voltage detecting circuit 13 functions to detect a high-voltage power supply and provides the feedback, protecting or detecting function to various ICs of the main circuit 32 to ensure that the power supply 10 performs a normal AC/DC conversion function during operation. Usually, the high-voltage detecting circuit 13 is constituted by a plurality of serially-connected resistors. What's worth mentioning is that the resistor layout of the high-voltage detecting circuit 13 will become more complicated if ICs with different functions are added to the position at which the PFC IC 111 is located. However, those resistors can be treated as a whole, e.g. the high-voltage detecting circuit 13.
Besides, when the power supply 10 stays in a standby mode, with an exception of an IC responsible for powering off the system operation (not shown), all ICs including the PFC IC 111 are shut off to save the power during the standby mode. Whereas, the high-voltage detecting circuit 13 purely constituted by the resistors is used to detect a high-voltage power supply, meaning that it will consume considerable power during the standby mode. Such drawback fails to meet the eager call in the commercial market attempting to lower the standby power loss of the electronic device.
A clear understanding is attainable by using actual values to carry out an estimation. Assume that the total resistance value is 1.2 MΩ for all resistors in the discharge circuit 12 and 1.5 MΩ for all resistors in the high-voltage detecting circuit 13, and the standby power loss of the IC in the main circuit 11 used to switch off the system is 70 mW:                (1) When the input terminal of the power supply 10 receives an 240V AC power, given the power equation relevant to resistor, P=V2/R, and the AC/DC conversion factor approximately equivalent to 1.4, the following calculations can be obtained:        the standby power loss of the discharge circuit 12 is:2402(V)/1.2(MΩ)=48(mW);        the standby power loss of the high-voltage detecting circuit 13 is:(240×1.4)2(V)/1.5(MΩ)=75(mW); and        as such, the standby power loss of the power supply 10 is:48(mW)+75(mW)+70(mW)=0.193(W).        (2) When the input terminal of the power supply 10 receives an 100V AC power, given the power equation relevant to resistor, P=V2/R, and the AC/DC conversion factor approximately equivalent to 1.4, the following calculations can be obtained:        the standby power loss of the discharge circuit 12 is:1002(V)/1.2(MΩ)=8.3(mW);        the standby power loss of the high-voltage detecting circuit 13 is:(100×1.4)2(V)/1.5(MΩ)=13(mW); and        As such, the standby power loss of the power supply 10 is:8.3(mW)+13(mW)+70(mW)=0.091(W).        
Currently, the standards in the commercial market demanding to lower the standby power loss of the electronic device tend to be strict, particularly in Japan, and the standby power loss shall be lower than 0.2 W for input AC power of 240V and lower than 0.1 W for input AC power of 100V respectively. As the aforementioned calculation result stands, the high-voltage detecting circuit 13 in the conventional power supply 10 as shown in FIG. 1 is the major reason that the prior art fails to lower the standby power loss.
To tackle the shortcoming of the power supply 10 as shown in FIG. 1, another solution of the prior art was brought up as shown in FIG. 2. Similarly, the power supply 20 is also composed of a main circuit 21, a discharge circuit 22 and a high-voltage detecting circuit 23, wherein the main circuit 21 includes a PFC IC 211. However, in comparison with FIG. 1, the input terminal of the discharge circuit 22 is connected with a relay 24 additionally. By means of the control of the relay 24 at the standby state, the standby power loss of the discharge circuit 22 and the high-voltage detecting circuit 23 in the power supply 20 can be entirely eliminated. As a result, the standby power loss is only 70 mW, which is the standby power loss of the IC in the main circuit 21 for powering off the system.
Although the power supply 20 in FIG. 2 has improved the disadvantage of the power supply 10 in FIG. 1, it still leads to the following drawbacks:                (1) The cost of the relay, which is relatively higher than that of the resistor, transistor switch, etc., impacts on the cost-down planning of the manufacturer.        (2) The reliability of the relay is an issue; in other words, while the total operation time thereof accumulates, the circuit joint thereof will have an aging effect, thereby deteriorating the system performance.        
For overcoming the drawbacks of the prior art, the present invention provides a novel high-voltage detection circuit for saving power at the standby mode, which brings about an improved design of the high-voltage detecting circuit.